Researchers from the CEA (French Atomic Energy Commission) study different hypotheses to understand the fertility of galaxies in the distant Universe. A new paradigm is emerging: a network of gas filaments may link the galaxies with each other and feed them with matter to create new stars.
This article is a translation of “Pourquoi les galaxies lointaines font-elles plus d’étoiles ?” by Timothée Froelich.
The researchers of the Cosmology and Galaxy Evolution Group (LCEG) of the CEA (French Atomic Energy Commission) have been studying galaxies for several years. They are trying to grasp why faraway galaxies give birth to more stars than those of the local Universe. Today, the measuring instruments, theoretical models and digital simulations let them reveal hidden characteristics of the universe. These new hypotheses could revolutionize our understanding of galaxy evolution.
Galaxy collisions: a deadlock
While studying the local Universe, the CEA’s astrophysicists came to question the accuracy of the traditional classification of galaxies, called the Hubble sequence. The MegaCam camera revealed that some elliptical galaxies that were supposed to be dead could still generate stars. Furthermore, the researchers demonstrated that “the collision of galaxies had a major role in determining their shape,” as Pierre-Alain Duc, astrophysicist in the Astrophysics Department of the CEA, summarized. Scientists use digital simulations to go back in time and recreate the galactic collision. Thus, they can explain the unusual shape of certain galaxies.
The research teams have, therefore, focused on these collisions of galaxies. The hypothesis was that they would encourage the concentration of matter, and thus star formation. In addition, galaxy collisions can cause the sudden, rapid creation of many stars, a phenomenon called a starburst. The galaxies of the distant Universe are much closer to each other and are more likely to collide. But is that the key factor that explains why the faraway galaxies are more fertile than those of the local Universe? Actually, no. Starbursts are minor phenomena that only account for 10 to 15% of the creation of stars. On the contrary, most of the time, the collisions would destroy matter more than they would merge it into new stars. Researchers were able to prove that the formation of stars in the distant Universe happens in the same way as in the local Universe. “The more molecular gas the galaxies contain, the more stars they give birth to,” states Mark Sargent, astrophysicist at the CEA, quite simply. Star formation is a universal phenomenon. “The universe turned out to be quite banal,” concludes David Elbaz, astrophysicist at the CEA.
The gas filament hypothesis
One observation remains unexplained. The concentration of stars in the universe contrasts with the proportion of available gas. Where does this matter come from? The CEA scientists hypothesize that galaxies are linked by catheter-like tubes of gas. This network of filaments may feed the galaxies and explain the continuous creation of stars.
But here’s the rub: even if, according to the theory and models, this hypothesis is admissible, the means of observation are not sufficient to validate it. The local Universe is too old for us to observe the filaments. Matter has diluted along with the expansion of the universe. Observing quasars, these very bright galaxies of the distant Universe, actually reveals the presence of matter, but not the shape it takes. The researchers rest their hopes on potential traces of matter resulting from star formation, like iron, in order to bring the filaments to light. As a matter of fact, atomic hydrogen, the main component of the universe, has not yet been detected. “Here, we have a new paradigm to test,” concludes David Elbaz.
To find out more:
Stars, NASA Science Astrophysics, NASA.
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